In recent years, Surface Acoustic Wave (hereinafter referred to as SAW) devices have been widely used as components for mobile communication terminals, in-car equipments and the like. For such SAW devices, there are strong requests such as downsizing, a high Q factor and fine frequency-temperature characteristics.
As a SAW device which can meet the above-mentioned requests, there is a SAW device using a ST-cut quartz crystal substrate. The ST-cut quartz substrate is a cut name of a quartz crystal substrate having a XZ′ plane obtained by rotating a XZ-plane counterclockwise from the crystalline Z-axis by 42.75° around the crystalline X-axis. A (P+SV) wave propagating in the crystalline X-axis direction called Rayleigh wave is utilized as a surface acoustic wave (hereinafter referred as “ST-cut quartz crystal SAW) in the ST-cut quartz crystal substrate. This SAW is utilized in the ST-cut SAW device. There are a wide range of applications of the ST-cut quartz crystal SAW device including a SAW resonator which is used as an oscillator and an IF filter disposed between an RF component and an IC in a mobile communication terminal.
One of the reasons why the ST-cut quartz crystal SAW device can realize a small-sized and high Q factor device is that the SAW reflection is efficiently utilized. Taking the ST-cut quartz crystal SAW resonator shown in FIG. 4 for example, the ST-cut quartz crystal SAW resonator a structure in which an interdigital transducer (hereinafter referred to as ITD) 102 having a plurality of electrode fingers interdigitating each other is disposed on a ST-cut quartz crystal substrate 101 and grating reflectors 103a, 103b reflecting SAW are respectively disposed at the both sides of the ITD 102. The ST-cut quartz crystal SAW propagates along the surface of a piezoelectric substrate and it is efficiently reflected by the grating reflectors 103a, 103b. Accordingly the energy of the SAW is sufficiently confined within the IDT 102 and this makes it possible to obtain the small-sized and high Q-factor device.
Meanwhile as an important factor in use of the SAW device, there is a frequency-temperature characteristic. In the case of the above-mentioned ST-cut quartz crystal SAW device, a first-order temperature coefficient of the frequency-temperature characteristic is zero so that the characteristic is represented by a quadratic curve when it is plotted. The amount of the variation in the frequency can be decreased significantly if the frequency-temperature characteristic is adjusted such that the peak temperature is to be located at the center of an operating temperature range. In this sense, it is commonly known that the ST-cut quartz crystal SAW device has a fine frequency stability.
Though the first-order temperature coefficient of the above-mentioned ST-cut quartz crystal SAW device is zero, the second-order temperature coefficient is −0.034 ppm/° C.2, which is relatively large. This can be a problem that the amount of the variation in the frequency becomes extremely large when the operating temperature range is expanded.
A saw device which can solve the above-mentioned problem was disclosed in Meirion Lewis, “Surface Skimming Bulk Wave, SSBW”, IEEE Ultrasonics Symp. Proc., pp. 744-752 (1977) and JP-B-62-016050. Referring to FIG. 5, a feature of this SAW device is that a cut angle “θ” of a rotated Y-cut quartz crystal substrate is set to about −50° rotated counterclockwise from the crystalline Z axis and the propagation direction of the SAW is set to the perpendicular direction (Z′-axis direction) with respect to the crystalline X axis. When the cut angle is expressed in Eular angle, the above-mentioned cut angle is (0°, θ+90°, 90°)=(0°, 40°, 90°). In this SAW device, a SH wave propagating just below the surface of the piezoelectric substrate is excited by the IDT, and the vibration energy is confined right under the electrodes. The frequency-temperature characteristic of the SAW device is expressed as a cubic curve and the variation in frequency becomes very small in the operating temperature range. In this sense, a fine frequency-temperature characteristic can be obtained.
However the SH wave generally propagates inside the substrate so that its reflection efficiency by the grating reflector is low compared with that of the ST-cut quartz crystal SAW that propagates along the surface of the piezoelectric substrate. For this reason, it is difficult to realize the small-sized high-Q SAW device. The above-mentioned prior literatures disclosed the application to a delay line that does not use the SAW reflection. However the prior literatures do not propose any application of the SAW reflection to devices and practical applications to the oscillation element and the filter element have been considered to be very difficult.
JP-B-01-034411 discloses a so-called multiple-pairs IDT type SAW resonator. Referring to FIG. 6, an IDT 112 having 800±200 pairs of electrodes is formed on a piezoelectric substrate 111 in which the cut angle “θ” of the rotated Y-cut quartz crystal substrate is set to about −50° and the propagation direction of SAW is set to the perpendicular direction (Z′-axis direction) with respect to the crystalline X-axis in the multiple-pairs IDT type SAW resonator. By providing a large number of the electrode pairs in the IDT 112, the multiple-pairs IDT type SAW resonator confines the SAW energy only by the reflection of the IDT 112 itself and without using the grating reflector, aiming to obtain a high Q factor.
However the multiple-pairs IDT type SAW resonator cannot confine the energy as efficiently as the SAW resonator with the grating reflector can. Accordingly the number of the pairs of IDT electrodes required to obtain a high level of the Q factor becomes as large as 800±200. This means that the device size exceeds that of the ST-cut quartz crystal SAW resonator and it cannot meet the request of downsizing.
According to JP-B-01-034411, it is said that the level of the Q factor can be raised by setting an electrode film thickness in 2% λ or larger and preferably equal to or smaller than 4% λ where “λ” is a wavelength of the SAW which is excited by the IDT. In this case when the resonance frequency is 200 MHz, the Q factor reaches the highest where the electrode film thickness is around 4% λ. However the highest value of the Q factor is only about 20,000, which is about the same as that of the ST-cut quartz crystal SAW resonator. This may be caused by the inefficient use of the reflection because the SAW is not sufficiently accumulated on the surface of the piezoelectric substrate where the film thickness lies in the range of 2-4% λ.
Considering the above-mentioned facts, in Japanese Patent Application No. 2004-310452, the inventor proposed a SAW device having a quartz crystal flat plate and an IDT that is made of Al or alloy containing Al as a main component and formed on the quartz crystal flat plate, and where the cut angle “θ” of the rotated Y-cut quartz substrate is set within a range of −64.0°<θ<−49.3°, more preferably −61.4°<θ<−51.1°, counterclockwise with the crystalline Z axis, the propagation direction of the SAW is set at 90°±5° with respect to the crystalline X axis, and an electrode film thickness “H/λ” which is the electrode film thickness normalized by the wavelength of the SAW in the IDT is set in a range of 0.04<H/λ<0.12, more preferably 0.05<H/λ<0.10. According to this invention, the wave which normally propagates inside the piezoelectric substrate can be concentrated to the surface of the substrate and the SAW can be sufficiently reflected by the grating reflector or the like. Thereby it is possible to realize the small-sized SAW device that has a better frequency-temperature characteristic and a higher Q factor compared to those of the hitherto known ST-cut quartz crystal SAW device.
Meanwhile, for obtaining a higher Q factor, it is important to reduce vibration loss as much as possible by confining the vibration energy of the SAW in the surface of the piezoelectric substrate. To achieve this, a peak frequency of a radiation conductance of the IDT has to be matched with a peak frequency of a reflection coefficient of the reflector. FIG. 7 shows a radiation conductance “G” and a reflection coefficient “|Γ|” of the reflector at the point where an electrode pitch “Lt” of the IDT is sized equal to an electrode pitch “Lr” of the reflector (Lt=Lr). It can be seen from the graph in the drawing that a peak frequency “ft” of the radiation conductance “G” of the IDT occurs at a different position from where a peak frequency “fr” of the reflection coefficient “|Γ|” of the reflector occurs. This indicates that a sufficient reflection state cannot be obtained at the center frequency of the IDT, which contributes to the deterioration of the Q factor.
The ratio of the electrode pitch “Lt/Lr” between the IDT and the reflector is needed to be adjusted in order to square the peak frequency “ft” with the peak frequency “fr”. The correction value of the ratio “Lt/Lr” varies according to the cut-angle of the piezoelectric substrate and the film thickness of the electrode therefore the ratio “Lt/Lr” should be appropriately selected depending on the design conditions, otherwise the peak frequency “ft” and the peak frequency “fr” cannot be matched and the Q factor is deteriorated. Accordingly the optimum value of the ratio “Lt/Lr” where the peak frequency “ft” square with the peak frequency “fr” should be examined for the SAW device disclosed in Japanese Patent Application No. 2004-310452.
The present invention has been achieved to eliminate the above-mentioned drawbacks and aims to obtain a high Q factor for the SAW device disclosed in Japanese Patent Application No. 2004-310452 by setting the ratio of the electrode pitch “Lt/Lr” between the IDT and the reflector appropriately and matching the peak frequency of the radiation conductance of the IDT with the peak frequency of the reflection coefficient of the reflector.